Pro-Russian activists in eastern Ukraine have decided to go ahead with an independence referendum on Sunday, despite a call from Russian President Vladimir Putin to postpone it.
The move was announced by separatist leaders after consulting supporters.
On Wednesday, Mr Putin called for a postponement to create the conditions necessary for dialogue.
Ukrainian authorities say they will disregard the results and that "anti-terror" operations will continue.
Millions of ballot papers have been prepared for the referendum.
The question put to voters is: "Do you support the act of proclamation of independent sovereignty for the Donetsk People's Republic?''
The decision to press ahead with the vote was announced by separatist leaders in the regions of Donetsk and Luhansk.
The leader in Donetsk, Denis Pushilin, said the decision had been unanimous.
"We just voice what the people want and demonstrate through their actions," he said.
A spokesman for the Kremlin said there was "little information" and that it needed to analyse the situation further.
The BBC's Steve Rosenberg in Moscow says Mr Putin may turn the separatists' decision to his advantage, claiming it is proof that Russia is not orchestrating events in eastern Ukraine - as claimed by the West.
Moscow has vowed to protect the rights of Ukraine's Russian-speaking population against what it calls an undemocratic government in Kiev.
Ukrainian authorities have rejected activist demands for greater autonomy and troops have been battling to regain official buildings occupied by rebels in the east.
The European Union weighed in on Thursday, warning that "such a vote could have no democratic legitimacy and would only further worsen the situation".
The separatists' decision to hold the referendum comes as a Pew Research Center poll released on Thursday shows that a strong majority of Ukrainians want their country to remain unified, even in the largely Russian-speaking east.
In related developments on Thursday:
- About 40 armed men attacked a border post in the Luhansk region but were repelled by guards, the Ukrainian border agency says
- In the south-eastern port city of Mariupol, the city hall changed hands for the third time in two days, with Ukrainian forces now back in control
- Russian Foreign Minister Sergei Lavrov spoke on the phone to US counterpart John Kerry and urged him to put pressure on Kiev "to begin a real de-escalation", the Russian foreign ministry said
- US Deputy Secretary of State William Burns reiterated the US position that Russia was stoking the unrest in eastern Ukraine, saying Moscow was heading down an "irresponsible path"
The US and the European Union have imposed sanctions against several Russian individuals and businesses and threatened wider measures if Moscow interferes further in eastern Ukraine.
Sunday's planned referendum was seen as a potential trigger for that.
Meanwhile Nato Secretary General Anders Fogh Rasmussen says there is no sign of a Russian troop withdrawal from the Ukrainian border, which Mr Putin announced on Wednesday.
Unrest in the south and east of Ukraine has worsened since Russia annexed the Crimean peninsula in March.
That followed the ousting of Ukraine's pro-Russian President Viktor Yanukovych in February by pro-Western protesters.